This invention relates to process vessels, for example, for use in the separation of a gas mixture.
One well known method of separating a gas mixture is by pressure swing adsorption. In pressure swing adsorption, a bed of molecular sieve adsorbent is employed to separate or extract relatively more of at least one component of a gas mixture admitted to the bed than of the remaining constitutent or constituents of the gas mixture. Accordingly, a gas enriched in the non-adsorbed constituents may be taken from the bed as a product. The bed is then regenerated by being subjected to a lower pressure than the one at which adsorption takes place.
Typically, two or more beds are employed and are operated out-of-phase with one another such that as one bed is adsorbing gas as another is being regenerated and a gaseous product is withdrawn from the plant continuously.
In known apparatus and plant for separating gas by pressure swing adsorption, each bed of molecular sieve is held in a cylindrical steel vessel. The vessels are fabricated at a vessel production site and are then transported typically by road to the site where the gas separation plant is to be installed. Owing to regulations concerning the size of loads which can be transported, there is a limit to the size of the vessels that can be transported. In the UK there are regulations prohibiting the transport of vessels or other loads having a diameter of more than 5 meters. Such regulations place a practical limit on the total capacity of the vessels, as they cannot be of unlimited height without running the risk of the molecular sieve particles being crushed and hence rendered less effective during operation of the plant. Accordingly, if it is desired to install a plant for the separation of gas mixture by pressure swing adsorption that has the capacity to produce gas at a relatively high rate, there is either the choice of fabricating suitably large vessels on the site where the plant is to be used, and this choice is usually impracticable, or, of using an increased number of smaller vessels. Sometimes, however, there is not the area available on the site of use for a large number of such vessels. Moreover, the use of a larger number of vessels containing molecular sieve imposes a need for a greater number of valves to be installed as well as more piping, thereby adding considerably to the cost of the plant.
Analogous problems arise in other processes, for example other gas separation processes using adsorbents, and processes using catalysts.